To switch an air supply duct and a return air duct by a space-saving method at a low cost without providing a bypass duct and to switch by a simple manual operation.

An air line 9 is selectively switched to either one of two states by a switching means 10. When the line 9 is switched to the one state, supply air is supplied to a hole 5 via the air supply duct 4. When the line 9 is switched to the other state, the supply air is supplied to the hole 5 via the return air duct 6. A box 11 is partitioned therein into four chambers 12A to 12D, which are respectively connected to a main air supply duct 2 extended from the air conditioner 1, a main return air duct 3 extended from the conditioner 1, the air supply duct 4 and the return air duct 6. The line 9 is switched by operating to open or close dampers 13A to 13D provided at partition walls for partitioning the box into the four chambers by the means 10.